In general, an electric vehicle includes: i) a charger for charging, from an external alternating current power source (socket), a main battery for driving a vehicle and ii) a DC-DC converter for charging, from the main battery, an accessory battery for driving accessories. The charger and the DC-DC converter form a circuit which uses a transformer for insulating a high-voltage circuit from a low-voltage circuit, where the high-voltage circuit is connected to the main battery while the low-voltage circuit includes the alternating current power source and the accessory battery. Recently, the person skilled in the art is trying to integrate the charger with the DC-DC converter. Patent document 1 discloses an electric power converter having such a structure that three windings, that is, a winding for a main battery, a winding for an alternating current power source and a winding for an accessory battery share a single transformer and are wound around a common core, where an electric power conversion is accomplished by way of the common core.